Just to report that I've had what I think is an occurrence of this problem this evening.
We had been watching a recording of Pointless in HD from a few days back. While we were doing that the box started recording Masterchef - The Professionals on BBC2 HD. When we finished Pointless SWMBO wanted to watch the Masterchef episode from the day before, but got the "scrambled" message when she tried to play it from the media list. So she went to the portal (the standard Humax portal) and watched it off iPlayer (in HD).
While she was watching that, this evening's episode of Masterchef finished and the box started to record Getting Rich in the Recession in Channel 4 HD, and IACGMOOH on STV HD. When Masterchef on iPlayer finished she tried to chase play the episode being recorded but it wouldn't play (I can't remember whether she got the "scrambled" message, or the same symptoms as with the later failed recordings). She then tried to chase play Getting Rich in the Recession, again with no luck. Finally she tried to watch an episode of The Life of Birds that I had recorded off BBC2 HD at the weekend, and again got the same result.
At this point the box became completely unresponsive to the remote. The webif was still working, but the box wouldn't respond to remote commands from the webif. Nor would it respond to any of the front panel buttons. In the end I had to power-cycle it.
Following the power-cycle, we could play the The Life of Birds episode with no problem, but the truncated recordings of How to Get Rich… and IACGMOOH both failed with a black screen for about ten seconds, after which you the media list appeared again. (Both programmes were listed with the "lightning bolt" icon but I believe this is normal if a recording is interrupted by a power outage anyway.)
Unfortunately I deleted those recordings before I found this thread, so they're not available for any diagnostic purposes (maybe I should have the undelete package installed!) The recording of this evening's episode of Masterchef is still on the box: it plays OK, but it is only 26 minutes long rather than the 67 minutes it should have been. That means that the recording ends at what would have been 20:24 ie well before the two subsequent recordings where supposed to begin (those programmes started at 21:00) and before I power-cycled the box (which I think was at around five past nine, based on the length of the recordings which were unplayable after the power-cycle).
For information, I'm running the 1.03.06 firmware with the 2.19 custom firmware.
I've run a few of the simpler diagnostics and all looks well.
It's reassuring to know that others' past experience suggests that this problem is infrequent and transient!